82 Mr, Wheat St one on Sound, [Aug. 



The sensation of sound can be excited by any of these bodies 

 when they oscillate sufficiently rapidly, either entire, or divided 

 into any number of parts in equilibrium with each other. The 

 laws of these subdivisions differ in the various phonics according 

 to their form and mode of connection or insulation ; and the 

 velocities of the oscillations, or degrees of tune, depend on the 

 form, dimensions, mode of connection, mode of division, and 

 elasticity of the body employed. The points of division in linear 

 phonics are called nodes, and the boundaries of the vibrating 

 parts of elastic surfaces are termed nodal lines. The parts at 

 which the oscillatory portions have their greatest excursions are 

 named centres of vibration ; these are always at the greatest 

 mean distances from the nodal points or lines. 



These mechanical oscillations are not, however, themselves 

 the immediate causes of sound ; they are but the agents in pro- 

 ducing in the bodies themselves, and in other contiguous sub- 

 stances, isochronous vibrations of certain particles varying in 

 magnitude according to the degree of tune. I convinced myself 

 of this important fact by the following simple experiments : I 

 took a plate of glass capable of vibrating in several different 

 modes, and covered it with a layer of water; on causing it to 

 .vibrate by the action of a bow, a beautiful reticulated surface of 

 vibrating particles commenced at the centres of the vibrating 

 parts, and increased in dimensions as the excursions were made 

 larger. When a more acute sound was produced, the centres 

 consequently became more numerous, and the number of coex- 

 isting vibratuig particles likewise increased, but their magnitudes 

 proportionably diminished. The sounds of elastic laraina3 are 

 generally supposed to be owing to the entire oscillations of the 

 simple parts as shown by Chladni, when, by strewing sand over 

 the sonorous plates, he observed the particles repulsed by the 

 vibrating parts, accumulate on the nodal lines, and indicate the 

 bounds of the sensible oscillations. Did no other motions exist 

 in the plate but these entire oscillations, the water laid on its 

 surface would, on account of its cohesion to the glass, show no 

 peculiar phenomena, but the appearances above described clearly 

 demonstrate that the oscillating parts consist of a number of 

 vibrating particles of equal magnitudes, the excursions of which 

 are greatest at the centres of vibration, and gradually become 

 less as they recede further from it, until they become almost null 

 at the nodal lines. 



To multiply these surfaces, and to observe whether the mag- 

 nitudes of these particles vary in different media, in a glass 

 vessel of a cylindric form, I superposed three immiscible fluids 

 of different densities; namely, mercury, water, and oil. On 

 producing the sounds corresponding with each mode of division, 

 I observed a number of vibrating parts, agreeing with the sound, 

 and showing similar appearances to the plate, formed on the 

 giU'faces of each of the fluids ; not the least agitation appeared 



